✨ feat(shared-auth-ui): add GuestWelcomeModal for guest onboarding

Add a unified welcome modal for guest mode that displays:
- App icon, name, and description from shared-branding
- Feature list of what guests can do (localized DE/EN)
- Warning about local-only data storage
- Login, Register, Help, and "Continue as Guest" buttons

New files:
- GuestWelcomeModal.svelte - The modal component
- guestWelcome.ts - localStorage utilities for tracking seen state

Integrated into: contacts, chat, todo, calendar, and clock apps

packages/shared-auth-ui/package.json

@@ -23,7 +23,8 @@
 	"peerDependencies": {
 		"svelte": "^5.0.0",
 		"@manacore/shared-auth": "workspace:*",
-		"@manacore/shared-icons": "workspace:*"
+		"@manacore/shared-icons": "workspace:*",
+		"@manacore/shared-branding": "workspace:*"
 	},
 	"devDependencies": {
 		"svelte": "^5.16.0",